FLIR Photon User Manual

52
Appendix C
approximately the amount of the sun’s
radiation that lies inside the visible light
spectrum.
Using the Stefan-Boltzmann formula
to calculate the power radiated by the
human body, at a temperature of 300 K
and an external surface area of approx.
 m

, we obtain 1 kW. This power loss
could not be sustained if it were not
for the compensating absorption of
radiation from surrounding surfaces at
room temperatures, which do not vary
too drastically from the temperature of
the body – or, of course, the addition of
clothing.
Non-blackbody Emitters
So far, only blackbody radiators and
blackbody radiation have been discussed.
However, real objects almost never
comply with these laws over an extended
wavelength region – although they may
approach the blackbody behavior in
certain spectral intervals. For example, a
certain type of white paint may appear
perfectly white in the visible light
spectrum, but becomes distinctly gray
at about 2 µm, and beyond 3 µm it is
almost black.
There are three processes that can
prevent a real object from acting like
a blackbody: a fraction of the incident
radiation α may be absorbed, a fraction ρ
may be reected, and a fraction τ may be
transmitted. Since all of these factors are
more or less wavelength dependent, the
subscript λ is used to imply the spectral
dependence of their denitions. Thus:
The spectral absorptance α•
λ
= the ratio
of the spectral radiant power absorbed
by an object to that incident upon it.
The spectral reectance ρ•
λ
= the ratio
of the spectral radiant power reected
by an object to that incident upon it.
The spectral transmittance τ•
λ
= the
ratio of the spectral radiant power
transmitted through an object to that
incident upon it.
The sum of these three factors must
always add up to the whole at any
wavelength, so we have the relation:
α
λ
+ ρ
λ
+ τ
λ
= 1
For opaque materials τ
λ
= 0 and the
relation simplies to:
α
λ
+ ρ
λ
= 1
Another factor, called the emissivity, is
required to describe the fraction ε of
the radiant emittance of a blackbody
produced by an object at a specic
temperature. Thus, we have the
denition: spectral emissivity ε
λ
= the
ratio of the spectral radiant power from
an object to that from a blackbody at the
same temperature and wavelength.
Expressed mathematically, this can
be written as the ratio of the spectral
emittance of the object to that of a
blackbody as follows:
W
λ0
ε
λ
=
_____
W
λb
Generally speaking, there are three types
of radiation sources, distinguished by the
ways in which the spectral emittance of
each varies with wavelength (Figures 12
and 13).
A blackbody, for which • ε
λ
= ε = 1
A graybody, for which • ε
λ
= ε = constant
less than 1
A selective radiator, for which • ε varies
with wavelength

FLIR Photon Specifications

General

Brand	FLIR
Model	Photon
Category	IP Camera
Language	English